The frequency of the signals that are dealt with in the field of electronics has recently reached 250 GHz, and the current status of high-speed electrical measuring technology is that the methods employed to observe these high-speed electric waveforms have not kept up with technical progress. Furthermore, advances in the miniaturization of circuit elements have led to a situation in which neither the temporal resolution nor the spatial resolution of electric measuring instruments has kept up with current technical progress.
An established and representative means of observing high-speed phenomena such as the high-speed operating state of microcircuit elements is the sampling oscilloscope. In addition, studies have recently been made of EO sampling, a technique which utilizes the electro-optic effect in electro-optic crystals (see Kamiya, T. and Takahashi, M., "Electro-optic sampling using semiconductor lasers," Oyo Butsuri, Vol. 61, No. 1, p. 30, 1992; and Nagatsuma, T., "Measurement of high-speed devices and integrated circuits using electro-optic sampling technique," IEICE Trans. Electron., Vol.E-76C, No. 1, January 1993). Given that it has become comparatively easy in the field of laser technology to obtain optical pulses in the subpicosecond range, optical sampling techniques seek to use such laser pulses for sampling electric signals. These techniques are faster than conventional electronic measurements, and can provide direct, non-contact measurements of electric potential at a desired point on a circuit under test, without leading out the signal. In other words, with this technique the electric pulses used for sampling in a sampling oscilloscope are replaced with optical pulses.
Another method for making high spatial resolution direct measurements of electric potential at a desired point is the electron-beam tester (see Plows, G., "Electron-beam probing," Semiconductors and Semimetals, Vol. 28 (Measurement of high-speed signals in solid-state devices), Chap. 6, p. 336, Ed. Willardson, R. K. and Beer, Albert C., Academic Press, 1990). Electron-beam testers are a powerful means of observing electric signals inside ICs for diagnosis and analysis of IC operation.
Scanning tunnelling microscopes and atomic force microscopes are among the devices which have recently undergone rapid development and come into widespread use as instruments for making high spatial resolution observations of the surface shape of objects under test. Because these devices are able to provide three-dimensional images at ultra-high spatial resolutions corresponding to the atomic scale, they are very well suited to the observation of the surface shape of semiconductor integrated circuits and the like. Bloom et al. have recently proposed a method for measuring the electric potential of an object under test using an atomic force microscope (AFM) (see Hou, A. S., Ho, F. and Bloom, D. M., "Picosecond electrical sampling using a scanning force microscope," Electronics Letters, Vol. 28, No. 25, p. 2302, 1992). In this method, a high-speed electronic circuit is used as the object being measured by an ordinary AFM, and a repulsive or attractive force is produced between the AFM probe and this object in accordance with the electric potential at the point being measured. This force gives rise to a minute displacement of the probe position. The method proposed by Bloom et al. involves detecting these minute displacements in order to measure changes with time in the electric potential at the point being measured.
However, the temporal resolution of a sampling oscilloscope is limited by the speed at which measurements can be carried out. This is dependent on a time constant which is determined by the width of the electric pulses used for sampling and by the resistance and capacitance of the measuring system. Moreover, because the signal being measured is led out from the measurement point via a cable or waveguide, it ends up being disturbed, with the result that there are problems regarding its reliability as well.
In the EO sampling technique, it is difficult to measure the absolute value of the signal, and there are practical problems relating to, among other things, the methods employed to provide high spatial resolution monitoring and control of probe position.
Electron-beam testers have a low temporal resolution and cannot be applied to the evaluation of ICs that use high-speed transistors. They also have the inconvenience of requiring a high vacuum as the measurement environment.
The temporal resolution of scanning tunnelling microscopes and atomic force microscopes has a limit that is set by the response speed of the probe, which is a mechanical system, and it is therefore difficult to use these devices for the measurement of high-speed electric waveforms.
Accordingly, a new means of measurement is required for the accurate evaluation of electric waveforms in large-scale integrated electronic circuits.
This situation may be looked at from another point of view as well. Namely, in prior art devices, only the pursuit of sampling speed has been regarded as important, and hardly any consideration has been given to positional control. The prior art will be explained with reference to FIG. 35, which is a block diagram of a prior art device. Object being measured 1 lies on testing stage 133, and the part of the object that is to be measured is set, by the operation of testing stage position controller 132, in the vicinity of probe tip 51, which is supported by probe arm 21. Next, after probe tip 51 has been brought into contact with object being measured 1, adjustment in the height direction is carried out by vertical position controller 130 which is formed from a piezoelectric element or the like, and the optimum measurement position is determined. Light is beamed onto probe tip 51 from light source 92. If an electric potential is present at the measurement site on object 1, the refractive index of the electro-optic crystal will change due to the electro-optic effect, whereupon the direction of polarization of the light reflected back from probe tip 51 will change from the direction it would have had if no electric potential had been present. The amount of change is detected by an optical system comprising wave plate 99, polarizer 97 and photodetector element 11, and is input to electric measuring instrument 60, which results in a measurement of the electric potential at the site being measured.
Prior art devices of this kind have the following problems. Namely, when the object being measured is an integrated circuit or the like where the thickness varies from place to place, every time the measurement site is changed it will be necessary to repeat the operations of bringing probe tip 51 into contact and positioning it, with the result that (1) measurements take a long time, and (2) the large mass of the part that holds probe tip 51 means that there is a strong chance of physically damaging the circuits of the object being measured.
Thus, in prior art devices, only the pursuit of sampling speed has been regarded as important, and hardly any consideration has been given to positional control.
The present invention has been devised in the light of this situation and meets the need for high temporal and spatial resolution measurement of high-speed electric waveforms at any measurement point on or in an integrated circuit. It is applicable to faster and more minute objects of measurement, and its purpose is to provide a more reliable high-speed electric measuring instrument and probe for the instrument, and an atomic force microscope which will serve as a high-speed electric measuring method and measuring instrument.
A further purpose of this invention is to provide an electro-optic measuring instrument which can perform high-precision position control.